Recent History of Astronomy

AST 111

The Geocentric Model

IT IS WRONG!

Fine. So what is it?

The Geocentric Model places Earth atthe center of the Universe. Everything

(Sun, Moon, stars, etc.) revolves aroundthe Earth.

The Geocentric Model

But it’s wrong. So why did it survivefor 2000 years?

Parallax and the Geocentric Model

• Things at different distances line up differently when you move around

• This change in angular separation is called parallax

– Hold your two index fingers at different distances and move your head around.

• Lack of observable parallax kept geocentric model alive.

Opposition to the Geocentric Model

• Sun-centered model proposed by Aristarchus

– Assertions:• If Earth orbits sun, angular separations of stars change• If they don’t, stars must be unrealistically far away

– Observation:• The angular separations of stars don’t seem to change

A “Conspiracy” in the Geocentric Model

• Venus maintains about the same brightness– Suggests it orbits Earth!– Should get dimmer as it gets farther away?

• This is a truly wretched coincidence.– As it gets farther, it gets dimmer.– BUT its phase increases (just like the Moon)

The Geocentric Model

• Greeks placed “Wandering Stars” at different distances

• Stars all on outer sphere

• Wandering stars “do their own thing in their own sphere”

Retrograde Motion

• “Wandering Stars” exhibited a strange feature

• Move forward, reverse briefly, resume going forward

• Notice that the size and brightness change

Geocentric Model

• “Wandering Stars” drove ancient astronomers up the wall, especially retrograde motion– Ultimately broke the geocentric model

• Was extremely difficult to fit to geocentric model– Became more and more complicated until simply

unreasonable

The Ptolemaic Model

• Greek knowledge of astronomy represented by the Ptolemaic Model

• “Wandering Stars” move around on small circles that rotate around a large circle

The Ptolemaic Model

• Little circle: Epicycle• Big circle: Deferent

The Ptolemaic Model

• Still did not predict “Wandering Star” motion accurately enough– Smaller circles added to the small circles– Positioned some of the larger circles off-center

• Accurate to “within a hand at arms length”

The Copernican Revolution

• This is where they started to get it right.

• Nicholas Copernicus born February 19, 1473 in Poland

• By then, tables of planetary motion from Ptolemaic model inaccurate

The Copernican Revolution

• Copernicus knew about Aristarchus’s sun-centered system

• Went farther with mathematical details

• Went from philosophical arguments to predictive geometry

It didn’t work very well.

The Copernican Revolution

• Was not very accurate

• Was made as complicated as Ptolemaic model to make reasonable predictions– He added epicycles

The Copernican Revolution

• It didn’t work because:

Copernicus held onto ancient belief that heavenly motion must occur in perfect circles.

Tycho Brahe

• Good data was hard to come by– No telescopes!

• Built a naked-eye observatory– Measured to within an

arcminute– 30 years of

measurements

Tycho Brahe

• Took large amounts of high quality data at his observatory

Tycho Brahe

• Aristotle : patterns of stars are “immutable” (unchanging)

• One blew up in 1572

• Tycho Brahe made parallax measurements– It was determined to be

as far away as the stars

Johannes Kepler

• Tycho Brahe’s apprentice

• Worked to match circular motions to Tycho’s data– Didn’t work– Trusted Tycho’s data

• Assumed sun-centered system

Johannes Kepler

Kepler deduced that planetary orbits arein the shape of ellipses, not circles.

Johannes Kepler

Rather than brushing aside small errorsto validate a preconceived notion,Kepler trusted the carefully taken

data and revolutionized astronomy.

Ellipses and Circles

Kepler’s Laws

• Newton was later able to derive Kepler’s Laws from more fundamental principles

Galileo Galilei

• Opponents of sun-centered system with elliptical orbits argued:– Non-circular orbits mean

celestial realm imperfect– Stars don’t shift position

as Earth orbits the Sun

• Galileo used the telescope to counter these arguments

Galileo Galilei

• Moons of Jupiter

Galileo Galilei

• The surface of the Sun:

Galileo Galilei

• Resolved individual stars in band of Milky Way:

Galileo Galilei

• Phases of Venus

Galileo Galilei

• Phases of Venus

Stellar Parallax

• Stellar parallax was finally observed in 1838

• Measured in arcseconds– 1/60 of the thickness of a fingernail at arm’s length

• Can be used to find the distance to nearby stars

• This marked the definitive end of the Earth-centered model.

“Wandering Stars”

• The “Wandering Stars” are PLANETS.

– (And planets are not stars!)

• They “wander” because they orbit the Sun and are much closer to Earth than stars